Optical frequency down-conversion and up-conversion are essentially the difference-frequency and the sum-frequency technologies. Compared with the traditional method by harmonic generation, the biggest advantage of the difference-frequency and sum-frequency synthesis technology systems is that the systems are portable and compact, and contains richer frequency components. Currently, this technology includes frequency spacing dichotomy, optical parametric oscillator method, nonlinear crystal optical method, laser diode four-wave mixing frequency method, optical frequency comb generator method, and so on. The frequency spacing dichotomy method and the optical frequency comb generator method are more characteristic in these methods. Both methods aim at dividing a large spacing of optical frequencies into a plurality of smaller frequency spacing in a certain relationship with a known frequency which generally can be measured by known means, thus an extremely convenient means is provided for the frequency detection technology.
From another perspective, if we can continue to move the measurable frequencies with small pitch and eventually get arbitrarily frequencies with large spacing, and these arbitrary optical frequencies can also be measured, it would be a very good idea. For example, a known frequency ωrf is known in advance, now in the vicinity of the original frequency ωc, via only implementations of known frequency ωrf multiplication or demultiplication several times, finally, any desired frequency ωc±nωrf can be achieved via the original frequency ωc. And at this time, the magnitude of each frequency can be shown up through a filter like a frequency “screen”. If this idea can really be realized, not only the technology can meet the requirements of the optical frequency measurement, but also any desired frequency of the light source can be obtained.
To realize this idea, the present invention discloses a tunable optical frequency converter based on a phase modulator. The invention will greatly simplify the structure of the frequency-tunable laser, significantly reduce the cost, miniaturize the presently large optical laboratory, since one such tunable optical frequency converter can replace dozens to hundreds of lasers, thus the integrated cost of the optical experiments can be substantially reduced, expected to be widely used in scientific research, industrial production and so on.